Traveling wave amplifier



Jan. 10, 1956 G. c. DEWEY 2,730,649

TRAVELLING WAVE AMPLIFIER Filed Feb. 4, 1950 2 Sheets-Sheet l INVENTOR CiOfiDO/Y C. DEWEY ATTORNEY Jan. 10, 1956 G. c. DEWEY 2,730,649

TRAVELLING WAVE AMPLIFIER F I d F b 4 1950 2 Sheets-Sheet 2 ie e INVENTOR GORDON C. DEWEY A ORNEY TRAVELING WAVE AMPLIFIER Gordon C. Dewey, New York, N. Y., assignor to International Telephone and Telegraph Corporation, a corporation of Maryland Application February 4, 1950, Serial No. 142,463

Claims. (Cl. 315--3.5)

This invention relates to high-frequency amplifiers of the travelling wave type and has for one of its objects the production of a more efficient amplifier having a very high power output.

Traveling Wave tubes heretofore known include a conductor either in the form of a helix or a series of interconnected baflies down the surfaces of which an electromagnetic wave can propagate. The phase velocity of the electromagnetic wave as measured along the axis of the conductor is small compared to the velocity of the Wave along the surface of the conductor. An electron beam source is arranged to direct a stream of electrons adjacent the conductor, and Where the conductor is in the form of a helix the beam is usually directed axially of the helix. The conductor path for the electromagnetic wave is so related to the axial path of the electrons that the electrons travel at a slightly higher velocity than the phase velocity of the wave. This difference in travel velocity results in part of the kinetic energy of the electrons being converted into radio frequency energy and the electromagnetic wave propagated in the direction of the electron flow is thereby amplified.

Such traveling wave amplifiers have a tendency to ring or oscillate under certain conditions of insertion loss and overall gain, particularly since some mismatch at the input and output terminations are for practical purposes, unavoidable. Reflections due to mismatch result in reverse radio frequency energy propagation in the tube. To prevent or minimize this reverse radio frequency energy propagation and thereby eliminate ringing or oscillation, it has been proposed heretofore to provide the conductor with a lossy or energy absorbing characteristic which presents a relatively high impedance to this reverse propagation of radio frequency energy. To accomplish this result, lossy material was either distributed equally along the conductor or concentrated over a short length centrally of the conductor. While this practice has proved effective in reducing the reverse propagation of radio frequency energy, it also limits the power output of the tube. It is customary, for example, to have insertion loss or attenuation in the wave propagating structure of the tube at least 15 db in excess of the net gain of the tube. For a typical helix tube with a gain of 35 db, this requires attenuation in excess of 45 db. It is natural that such large attenuation, while not seriously affecting the gain of the tube, will have a very pronounced effect on the power output andefficiency of the tube.

It is a further .object of the invention, therefore, to provide a travelling wave amplifier with the lossy material so located along the axial length of the tube. as to maintain a high gain and insertion loss relationship and yet also a high power output similar to that obtainable by a substantially loss-free or non-attenuating traveling Wave 7 tube.

The invention, briefly, provides a high frequency wave transmission means having a lossy characteristic adjacent the emmiter electrode and a substantially loss-free char- 1 United States Patent Q acteristic adjacent the collector electrode. In other words, the guide for transmitting the electromagnetic wave has associated therewith lossy material distributed along the length of the conductor from a point closely adjacent the emitter electrode to about five-eighths of the length thereof. This uniform lossy structure of the transmission means preferably extends along the initial and middle sections of the tube and is substantially eliminated from the output or high level section of the tube. For best results I find the loss-free section of the tube should be from about 5 to 15 guide wavelengths long and should present as little loss as possible. By thus locating the lossy material along the initial and middle sections of the tube, the efiiciency of the tube, as compared to the same type having the lossy material equally distributed throughout the length thereof, has been more than doubled with a corresponding increase in power output.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood, by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a longitudinal cross-sectional view taken along line 1-1 of Fig. 2 of one form of travelling wave tube incorporating the present invention.

Fig. 2 is a cross-sectional view taken along lines 22 of Fig. 1;

Fig. 3 is a longitudinal cross-sectional view of another form of travelling wave tube embodying the present invention; and

Figs. 4 and 5 are cross-sectional views taken along lines 44 and 5-5, respectively, of Fig. 3.

Referring to Figs. 1 and 2 of the drawing, a form of travelling wave amplifier tube having a helically coiled conductor for transmission of the electromagnetic wave is shown. The tube includes an envelope 1 enclosing an electron gun 2, focusing electrodes 3 and 4, a tightly helically wound conductor 5, and a collector electrode 6. The axis of the helix 5 is aligned with the electron gun 2 which is indirectly heated by coil 7, lead 3 and 9 of the coil being connected across a battery 10. A lead 11 from the electron gun is connected to the negative side of a high tension source 12. Leads 13 and 14 from the focusing electrodes 3 and 4 are connected to low-voltage points of the source 12. Leads 15 and 16 from the collector electrode 6 and the helix 5 are also connected to the source 12.

The helix 5 is connected at each end to metal cylinders 17 and if each being arranged coaxially about the beam path and adapted to fit inside the glass envelope. The helix 5 is connected to the cylinders by conductor stubs l9 and 20. This helix serves as a guide for the electromagnetic energy axially of the tube. A rectangular input wave guide 21 encircles the envelope at the location of the stub 19 and a similar rectangular output wave guide 22 encircles the envelope at the location of the stub 20, the proportioning of guides 21 and 22 and the stubs l8 and 19 are such as to provide proper match over at least the operating frequency range between the wave input guide and the helix at the input of the tube and the helix and the output guide at the output of the tube. A solenoid 23 is arranged coaxially about the helix 5, for focusing the beam therethrough.

The foregoing description is typical of the helix type of travelling wave amplifier. In accordance with the present invention, the envelope is provided with a plurality of indentations or inwardly disposed ribs or flutes as indicated at 24, 25 and 26. These indentations are equally spaced circumferentially of the tube and preferably extend for the major length of the tube substantially coextensive with the helix 5. While the indentations may be made short and spaced apart along the length of the tube, it is preferred to extend them as indicated in Figs. 1 and 2. The innermost surfaces of the indentations or ribs 24, 25 and 26 are shaped and if need be ground to conform with the curvature of the helix. These indentations, thus shaped are used to support the helix and maintain it in true alignment coaxially about the beam path.

Lossy material 27, such as aquadag, Nichrome, metal oxides, carbon or other materials of lossy characteristic, is coated in a thin film on the outside of the tube within the recesses formed by the indentations 24, 25 and 26. The lossy material is located in these recesses lengthwise of the tube from a point adjacent the input connection 21 to about five-eighths the distance toward the collector electrode. While this location of the lossy material on the outer surface of the envelope is preferred, it will be understood that the lossy material may be placed on the inner surface of the envelope, or actually included as a part of the conductor itself. The reason for placing the lossy material on the outer surface of the envelope is that it may be easily applied, removed and relocated, as desired. Furthermore, the recesses formed by the indentation protect the film of lossy material applied thereto, since in handling, these recessed portions of the envelope are protected from the hands of the operator.

Experiment shows that in order to obtain optimum results in eficiency and power output the distribution of lossy material should be limited to the initial and central sections of the tube with the final section maintained as loss-free as possible. The loss-free section should be between 5 to about guide wavelengths long to obtain the best results. It is known that attenuation in a wave propagating structure reduces the phase velocity of the structure by a percentage equal to a/fi 100%, where a is the attenuation constant of the structure in nepers per meter and 5 is the propagation constant in radians per meter. It may be desirable, therefore, to modify the phase velocity of the structure in the output or lossfree section so as to maintain substantially constant phase velocity along the transmission structure as required for the operation of the travelling wave tube amplifier. To accomplish the required small change in phase velocity, the pitch of the helix or the geometry of the periodic wave guide structure may be varied. It is also desirable to join the loss-free section to the lossy section as a continuation thereof so as to minimize reflections.

The reasons for the improved efiiciency and higher power output of tubes incorporating my invention are not fully understood but it is believed that the improved results may be explained as follows. By locating the lossy material along the initial and central portions of the high frequency transmission guide with the final portion of the guide as loss-free as possible, the reverse flow of high frequency energy is substantially completely damped where such energy is the weakest and the propagated wave along the guide is unimpeded along the final portion of the guide where the more pronounced electron bunching, and therefore amplification, occurs and where the energy level is the greatest. By maintaining the final portion, that is, a length of from 5 to 15 guide wavelengths at the output end of the tube as loss-free as possible, the maximum energy transfer from the electron flow to the electromagnetic wave is obtained. The location of the lossy material along the initial and central portions of the guide has little effect on the energy transfer from the electron fiow since the bunching of electrons is initiated along those portions and has not reached the maximum proportions until the electrons have reached the final section of the tube. It will be understood, of course, that some variation of the extent to which the lossy material may be applied along the guide may be made without materially altering the efficiency and output of the tube. In this respect, it is to be understood that while I have mentioned a length of five-eighths over which the lossy material may be distributed, this length may be varied without departing from the invention. It is important, however, that the lossy material be distributed over the initial portion and at least part of the central portion of the guide and that the final or latter portion of the guide be maintained as loss-free as possible.

Referring to Figs. 3 to S of the drawing, a travelling wave amplifier tube of the form disclosed in my copending application, Serial No. 58,293, filed November 4-, 1948, now Patent No. 2,643,353, is shown. This tube includes an envelope 28 which encloses an electron gun 29, focusing electrode 30, cylindrical coaxial wave guide or outer conductor 31 and collector electrode 32. Extending axially of the outer conductor 31 is an inner conductor 33 which carries a plurality of plane conducting discs or bafiles 34, 35. The gun 29 and the focusing electrode 30 are annular in shape so as to emit a cylindrical shaped beam of electrons. The envelope is surrounded by a coil 43 for the provision of a magnetic field parallel to the path of the beam between the gun and the collector electrode.

The coaxial travelling wave tube is coupled to a coaxial input line 36 which extends through the end of the tube 28 and is connected to the outer conductor 31 by means of a spider 37. The inner conductor 33 is coupled to the inner conductor 38 of the input line by a slug 39, the dimensions of which are so chosen that the combination transmission effect of the changes in diameter and length of the slug presents a good impedance match over a broad band. A similar transfer coupling slug connects the output end of conductor 33 to the output of the inner condoctor 40 of the output line. Likewise a spider 41 couples the output end of the cylinder 31 to the outer conductor 42 of the coaxial line. The spacing of the battles 34, 35 from each other and the outer conductor is greatly exaggerated in the drawings but in actuality the spacing is very small. The discs 34 which extend over the initial and central portion of the conductor 33 include high-loss material. The high loss material may be coated on a base metal or included as a part of the composition of the disc. The discs 35 which extend over the final portion of the conductor 33 are as loss-free as possible. These discs are preferably made of molybdenum plated with silver or copper. The base metal of the discs 34 may also be of molybdenum but preferably are of a different metal, such as Nichrome. This type of tube provides for a much larger electron beam than the helix type of tube and, therefore, a larger power output is obtainable.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention.

I claim:

1. A travelling wave amplifier comprising an electron emitter electrode and an electron collector electrode spaced apart along a given axis to define an electron path therebetween, transmission means comprising a conductor extending from a point adjacent said emitter electrode to a point adjacent said collector electrode, said conductor being of a character to provide for propagation of an electromagnetic wave with a component thereof traveling axially along said path at substantially the same velocity as the electron beam, the forward half of said conductor extending from a point adjacent said emitter electrode having a lossy characteristic for substantially the entire length thereof while the rearward half of said conductor extending to a point adjacent said collector electrode has a substantially lossless characteristic for substantially the entire length thereof.

2. A travelling wave amplifier according to claim 1, wherein the conductor comprises a rod having conducting baffies spaced therealong.

3. A travelling wave amplifier comprising an electron emitter electrode and an electron collector electrode spaced apart along a given axis to define an electron path therebetween, transmission means disposed along said electron path for propagation of an electromagnetic wave along said path, the forward half of said transmission wards said electron path and the lossy characteristic of said transmission line includes lossy material disposed in said indentations.

4. A travelling wave amplifier according to claim 3, wherein said transmission means includes a conductor for high frequency waves, said conductor being at least partially supported by the inner envelope surfaces defined by said indentations.

5. A travelling wave amplifier according to claim 4, wherein said conductor is a helix and said indentations engage at least certain of the turns of the helix.

6. A travelling wave amplifier according to claim 5, wherein the indentations are elongated lengthwise of said envelope to present a plurality of inwardly disposed ribs spaced circumferentially of said envelope to engage turns of said helix thereby maintaining said helix coaxially of said electron path.

7. A traveling wave amplifier comprising electron emitter and collector electrodes spaced apart along a given axis to define an electron path therebetween, an elongated envelope disposed about said path having longitudinal flutes disposed lengthwise thereof to provide a plurality of inwardly disposed elongated ribs spaced circumferentially about said envelope, and a helical conductor disposed in supported relation by said ribs, said helical conductor extending from a point adjacent said emitter electrode to a point adjacent said collector electrode, at least certain of the flutes of said envelope contain lossy material for at least a portion of the length thereof.

3 8. A traveling Wave amplifier according to claim 7,

wherein the lossy material is disposed in said flutes for a distance corresponding substantially to the initial half of said helical conductor.

9. A travelling Wave amplifier comprising an electron emitter electrode and an electron collector electrode spaced apart along a given axis to define an electron path therebetween, transmission means comprising a conductor extending from a point adjacent said emitter electrode to a point adjacent said collector electrode, said conductor being of a character to provide for propagation of an electromagnetic Wave with a component thereof travelling axially along said path at substantially the same velocity as the electron beam, substantially five-eighths of said conductor measured from adjacent said emitter electrode having a lossy characteristic while the remaining portion towards said collector electrode has a substantially lossless characteristic.

References Cited in the file of this patent UNITED STATES PATENTS 1,581,520 Schwerin Apr. 20, 1926 1,957,538 Jensen May 8, 1934 2,064,469 Haeff Dec. 15, 1936 2,122,538 Potter July 5, 1938 2,300,052 Lindenblad Oct. 27, 1942 2,367,295 Llewellyn Jan. 16, 1945 2,413,608 DiToro Dec. 31, 1946 2,541,843 Tiley Feb. 13, 1951 2,575,383 Field Nov. 20, 1951 2,584,597 Landauer Feb. 5, 1952 2,584,802 Hansell Feb. 5, 1952 2,585,582 Pierce Feb. 12, 1952 2,602,148 Pierce July 1, 1952 2,653,270 Kornpfner Sept. 22, 1953 OTHER REFERENCES Article by Hollenberg, pages 52-58, Bell System Tech. Journal, January 1949. Copy in U. S. Patent Otfice Scientific Library.

Article by Kompfner, pp. 124-127, Proc. I. R. E., February 1947. Copy in Patent Oflice Science Library. 

